Increasing concern for the flame retardancy of residential and commercial construction elements has spurred considerable development activity in the recent past. Thus, important steps have been taken to render plastic building components flame retardant and the work has also been extended to include cellulosic materials employed for construction purposes. As a result of the work performed, the patent literature is replete with numerous processes for the flame proofing of cellulosic boards, such as hardboards, fiberboards and/or particle-boards. To achieve the desired flame retardance in these materials, the prior art recommends incorporation of various inorganic and organic flameproofing agents and also combinations of these in the boards. For example, as early as 1933, U.S. Pat. No. 1,907,711 (Becher) recommended the addition of alumina hydrate [Al(OH).sub.3 ] as fireproofing agent to boards made from cellulosic materials, such as wood pulp. The incorporation of alumina hydrate in the wood pulp resulted in boards which exhibited a high degree of fire resistance and possessed reduced afterglow characteristics.
Prior art attempts to further improve the fire retardance of cellulosic boards included the addition of boron-containing compounds to cellulosic boards, for example, U.S. Pat. No. 2,611,694 (Becher) mentions the addition of borax to cellulosic boards. Incorporation of a boric acid-borax mixture in laminated wood to impart flame retardancy has been reported in a recent Canadian patent (U.S. Pat. No. 971,049 -- Perlus et al). Addition of such highly water-soluble boron salts as flame-retarding agents to cellulosic boards involves leachability problems, particularly when the boards are exposed to rain or moisture of condensation, which can cause gradual reduction in the flame retardancy of these boards. The pertinent prior art also includes U.S. Pat. No. 3,865,760 to Pitts et al (1975), where a plastic composition, particularly a foamed or solid rubber product, is rendered flame retardant by incorporation of a hydrated, boron-containing material of reduced water-solubility, such as colemanite (Ca.sub.2 B.sub.6 O.sub.11 .multidot.5H.sub.2 O) or ulexite (NaCaB.sub.5 O.sub.9 .multidot.8H.sub.2 O). The boron-containing agent, as mentioned in this patent, can be used either alone or in combination with other flame retardants, for example, hydrated alumina. This reference indicates that colemanite, when added in a 1.5:1 weight ratio to rubber, was capable of increasing the oxygen index of the mixture to about 29, while alumina trihydrate, when incorporated without colemanite, also in a 1.5:1 weight ratio of Al(OH).sub.3 to rubber, provided an oxygen index of 30. A 1:1 mixture by weight of colemanite and Al(OH).sub.3, when used to flame-retard a rubber composition in a colemanite-alumina trihydrate: rubber weight ratio of 1.5:1, resulted in an oxygen index of 31, which is only slightly higher than the value attained with colemanite alone, or with colemanite-free alumina hydrate.
Thus, use of a mixture of the hydrated boron-containing mineral with alumina hydrate, did not significantly improve the flame retardancy of the rubber composition over the results obtained with the individual components of the flame-retarding mixture and at best provided only an economic advantage in employing, for example, a less expensive component in the flame-retarding agent.
The results obtained with flame-retardant rubber compositions with either colemanite, ulexite or alumina hydrate, or with mixtures of these, when applied to plastics, such as rubber, may provide overall satisfactory results. However, when either the individual components or the mixtures of the individual components are incorporated in cellulosic boards, such as hardboards or particle-boards, in amounts and ratios described in the above-referred to Pitts et al reference, a cellulosic board is obtained which while exhibiting suitable flame retardance, although high afterglow, lacks important physical strength properties required from these boards. This is mainly due to the large quantity of flame-retarding agent prescribed by the Pitts et al reference. On the other hand, when reduced quantities of such flame-retarding compositions are added to plastic compositions, they fail to impart the necessary flame retardancy to these compositions, demonstrating the requirement for high flame-retarding agent loading in order to obtain the desired results.
It has now been surprisingly discovered that when a mixture of ulexite and alumina trihydrate is incorporated in cellulosic boards in controlled quantities, which are significantly lower than those referred to in the prior art, boards of excellent flame retardancy and satisfactory physical properties can be obtained. The ulexite-alumina trihydrate flame-retarded boards made by the present discovery are free of afterglow, meet presently defined Class I fire ratings and, in addition, satisfy all physical strength requirements set forth by existing standards. The unobvious and surprising nature of the present invention becomes even more pronounced when it is considered that neither ulexite, nor alumina trihydrate alone, is capable of producing the same flame-retarding effect in cellulosic boards, as the mixture of these at the same loading. This fact clearly indicates the unexpected synergistic effect of ulexite-alumina trihydrate mixtures on the flame retardancy of cellulosic boards.